Lithology

Distribution by Scientific Domains

Kinds of Lithology

  • different lithology


  • Selected Abstracts


    Lithology and fluid prediction from amplitude versus offset (AVO) seismic data

    GEOFLUIDS (ELECTRONIC), Issue 4 2003
    D. J. Davies
    Abstract Seismic reflection data as used in the oil industry is acquired and processed as multitrace data with source-receiver offsets from a few hundred metres (short offset) to several kilometres (long offset). This set of data is referred to as ,pre-stack'. The traces are processed by velocity analysis, migration and stacking to yield a data volume of traces with ,zero-offset'. The signal-to-noise enhancement resulting from this approach is very significant. However, reflection amplitude changes in the pre-stack domain may also be analysed to yield enhanced rock physics parameter estimates. Pre-stack seismic data is widely used to predict lithology, reservoir quality and fluid distribution in exploration and production studies. Amplitude versus offset (AVO) data, especially anomalous signals, have been used for decades as indicators of hydrocarbon saturation and favourable reservoir development. Recently, enhanced quantification of these types of measurement, using seismic inversion techniques in the pre-stack domain, have significantly enhanced the utility of such measurements. Using these techniques, for example, probability of the occurrence of hydrocarbons throughout the seismic data can be estimated, and as a consequence the many pre-stack volumes acquired in a three-dimensional (3D) can be survey, reduced to a single, more interpretable volume. The possibilities of 4D time lapse observation extend the measurements to changes in fluid content (and pressure) with time, and with obvious benefits in establishing the accuracy of dynamic reservoir models and improvements in field development planning. As an illustration, recent results from the Nelson Field (UK North Sea), are presented where we show the method by which probability volumes for oil sands may be calculated. The oil,sand probability volumes for three 3D seismic datasets acquired in 1990, 1997 and 2000 are compared and production effects in these data are demonstrated. [source]


    Lithology and hydrocarbon mapping from multicomponent seismic data

    GEOPHYSICAL PROSPECTING, Issue 2 2010
    Hüseyin Özdemir
    ABSTRACT Elastic rock properties can be estimated from prestack seismic data using amplitude variation with offset analysis. P-wave, S-wave and density ,reflectivities', or contrasts, can be inverted from angle-band stacks. The ,reflectivities' are then inverted to absolute acoustic impedance, shear impedance and density. These rock properties can be used to map reservoir parameters through all stages of field development and production. When P-wave contrast is small, or gas clouds obscure reservoir zones, multicomponent ocean-bottom recording of converted-waves (P to S or Ps) data provides reliable mapping of reservoir boundaries. Angle-band stacks of multicomponent P-wave (Pz) and Ps data can also be inverted jointly. In this paper Aki-Richards equations are used without simplifications to invert angle-band stacks to ,reflectivities'. This enables the use of reflection seismic data beyond 30° of incident angles compared to the conventional amplitude variation with offset analysis. It, in turn, provides better shear impedance and density estimates. An important input to amplitude variation with offset analysis is the Vs/Vp ratio. Conventional methods use a constant or a time-varying Vs/Vp model. Here, a time- and space-varying model is used during the computation of the ,reflectivities'. The Vs/Vp model is generated using well log data and picked horizons. For multicomponent data applications, the latter model can also be generated from processing Vs/Vp models and available well data. Reservoir rock properties such as ,,, ,,, Poisson's ratio and bulk modulus can be computed from acoustic impedance, shear impedance and density for pore fill and lithology identification. , and , are the Lamé constants and , is density. These estimations can also be used for a more efficient log property mapping. Vp/Vs ratio or Poisson's ratio, ,, and weighted stacks, such as the one computed from ,, and ,/,, are good gas/oil and oil/water contact indicators, i.e., pore fill indicators, while ,, mainly indicates lithology. ,, is also affected by pressure changes. Results from a multicomponent data set are used to illustrate mapping of gas, oil and water saturation and lithology in a Tertiary sand/shale setting. Whilst initial log crossplot analysis suggested that pore fill discrimination may be possible, the inversion was not successful in revealing fluid effects. However, rock properties computed from acoustic impedance, shear impedance and density estimates provided good lithology indicators; pore fill identification was less successful. Neural network analysis using computed rock properties provided good indication of sand/shale distribution away from the existing wells and complemented the results depicted from individual rock property inversions. [source]


    Biophysical and human influences on plant species richness in grasslands: Comparing variegated landscapes in subtropical and temperate regions

    AUSTRAL ECOLOGY, Issue 3 2001
    S. Mcintyre
    Abstract A survey of grassy woodlands in the Queensland subtropics was conducted, recording herbaceous species richness at 212 sites on three properties (2756 ha). A range of habitats typical of cattle grazing enterprises was sampled and site variables included lithology, slope position, tree density, soil disturbance, soil enrichment and grazing. Results were compared with a previously published survey of temperate grasslands. Lithology, slope position and tree density had relatively minor effects on plant species richness, although in both surveys there was some evidence of lower species richness on the more fertile substrates. Soil disturbance and soil enrichment significantly reduced the richness of native species in both surveys, while exotic species were insensitive (subtropics) or increased (temperate) with disturbance. Rare native species were highly sensitive to disturbances, including grazing, in the temperate study. Although some trends were similar for rare species in the subtropics, the results were not significant and there were complex interactions between grazing, lithology and slope position. Grazing did not have a negative effect on native species richness, except in the closely grazed patches within pastures, and then only on the most intensively developed property. At the scale recorded (30 m2), the native pastures, roadsides and stock routes sampled in the subtropics appear to be among the most species-rich grasslands ever reported, both nationally and globally. Native species richness was approximately 50% higher than the temperate survey figures across all the comparable habitats. While there are no clear reasons for this result, potential explanations are proposed. [source]


    A minimum sample size required from Schmidt hammer measurements

    EARTH SURFACE PROCESSES AND LANDFORMS, Issue 13 2009
    Tomasz Niedzielski
    Abstract The Schmidt hammer is a useful tool applied by geomorphologists to measure rock strength in field conditions. The essence of field application is to obtain a sufficiently large dataset of individual rebound values, which yields a meaningful numerical value of mean strength. Although there is general agreement that a certain minimum sample size is required to proceed with the statistics, the choice of size (i.e. number of individual impacts) was usually intuitive and arbitrary. In this paper we show a simple statistical method, based on the two-sample Student's t -test, to objectively estimate the minimum number of rebound measurements. We present the results as (1) the ,mean' and ,median' solutions, each providing a single estimate value, and (2) the empirical probability distribution of such estimates based on many field samples. Schmidt hammer data for 14 lithologies, 13,81 samples for each, with each sample consisting of 40 individual readings, have been evaluated, assuming different significance levels. The principal recommendations are: (1) the recommended minimum sample size for weak and moderately strong rock is 25; (2) a sample size of 15 is sufficient for sandstones and shales; (3) strong and coarse rocks require 30 readings at a site; (4) the minimum sample size may be reduced by one-third if the context of research allows for higher significance level for test statistics. Interpretations based on less than 10 readings from a site should definitely be avoided. Copyright © 2009 John Wiley & Sons, Ltd. [source]


    Rock albedo and monitoring of thermal conditions in respect of weathering: some expected and some unexpected results

    EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2005
    Kevin Hall
    Abstract Broadly speaking, there is, at least within geomorphic circles, a general acceptance that rocks with low albedos will warm both faster and to higher temperatures than rocks with high albedos, reflectivity influencing radiative warming. Upon this foundation are built notions of weathering in respect of the resulting thermal differences, both at the grain scale and at the scale of rock masses. Here, a series of paving bricks painted in 20 per cent reflectivity intervals from black through to white were used to monitor albedo-influenced temperatures at a site in northern Canada in an attempt to test this premise. Temperatures were collected, for five months, for the rock surface and the base of the rock, the blocks being set within a mass of local sediment. Resulting thermal data did indeed show that the dark bricks were warmer than the white but only when their temperatures were equal to or cooler than the air temperature. As brick temperature exceeded that of the air, so the dark and light bricks moved to parity; indeed, the white bricks frequently became warmer than the dark. It is argued that this ,negating' of the albedo influence on heating is a result of the necessity of the bricks, both white and black, to convect heat away to the surrounding cooler air; the darker brick, being hotter, initially convects faster than the white as a product of the temperature difference between the two media. Thus, where the bricks become significantly hotter than the air, they lose energy to that air and so their respective temperatures become closer, the albedo influence being superceded by the requirement to equilibrate with the surrounding air. It is argued that this finding will have importance to our understanding of weathering in general and to our perceptions of weathering differences between different lithologies. Copyright © 2005 John Wiley & Sons, Ltd. [source]


    An Ordovician age for the Muggort's Bay Lower Palaeozoic inlier, County Waterford, Ireland,the southernmost exposure of the Irish Caledonides

    GEOLOGICAL JOURNAL, Issue 5 2005
    P. M. Brück
    Abstract The most southerly exposed Lower Palaeozoic strata in Ireland occur on the southwest coast of County Waterford along a 2.5,km long coastal section at Muggort's Bay where they are surrounded by Devonian rocks. Five formations can be distinguished which, in ascending order, are: the Ballycurreen, Carrickbrean, Rathnameenagh, Moanbrack and Killinoorin formations. The total thickness of the succession is over 1800,m. No macrofossils are present, but the lithologies are largely fine-grained turbidites and subordinate volcanic rocks which closely resemble the Ribband Group seen elsewhere in southeast Ireland and have previously therefore been classified with it. Palynological analysis was undertaken on 25 samples collected from Muggort's Bay, of which eight were productive. Diagnostic microfossils, comprising acritarchs, chitinozoans and scolecodonts, indicate an Early to Middle Ordovician age for both the Rathnameenagh and the Moanbrack formations. These ages confirm that the strata are part of the Ribband Group which elsewhere has been biostratigraphically dated as ranging from Mid-Cambrian to Mid-Ordovician. Reworked mid-Middle Cambrian acritarchs occur in the Moanbrack Formation and reworked late Middle to early Late Cambrian acritarchs in the Rathnameenagh Formation. Despite generally poor preservation of the organic matter, some 20 acritarch species have been distinguished. Among these, three species belong to the herein revised genus Retisphaeridium for which an emended diagnosis is proposed together with two new combinations, Retisphaeridium capsulatum (Jankauskas, 1976) Vanguestaine nov. comb. and Retisphaeridium pusillum (Moczydlowska, 1998) Vanguestaine nov. comb. Copyright © 2005 John Wiley & Sons, Ltd. [source]


    On the structural age of the Rhoscolyn antiform, Anglesey, North Wales

    GEOLOGICAL JOURNAL, Issue 2 2004
    Hossein Hassani
    Abstract In the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre-scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller-scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second-generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small-scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first-order analysis of the finite strains from around the large-scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast-verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest-verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd. [source]


    Localized ductile thrusting north of the Variscan Front, Ross Island, southwest Ireland

    GEOLOGICAL JOURNAL, Issue 1 2003
    Paul A. M. Nex
    Abstract Two thrusts occur on Ross Island: the Head of Ross Thrust and the more southerly Ross Island Thrust. These lie to the north of the Killarney,Mallow Fault (KMF), the boundary frequently interpreted as the Variscan Front. The Ross Island Thrust, exposed in outcrop and in seven borehole cores, has emplaced dark blue,grey limestones of the Courceyan Ballysteen Formation over pale grey,brown Rockfield Limestone Formation of Chadian,Holkerian age. These lithologies at Ross Island exhibit a continuum of deformation at both the micro- and macro-scale, beginning with the generation of a spaced cleavage, formed during layer parallel shortening, that was subsequently rotated into parallelism with fold axial planes. Extensional microstructures are predominant in thin section and are associated with attenuation of the fold limb. Calcite veins are also attenuated and lie parallel to a mylonitic fabric close to the thrust plane. Lithological boundaries, the prominent pressure solution cleavage and the southerly dipping limb of an asymmetrical antiform are all parallel and form a composite planar anisotropy. This has controlled the location of the ductile Ross Island Thrust, which formed during the attenuation and shearing of a common fold limb. Ductile thrusts within the limestones at Ross Island contrast with the reactivation of basin-margin extensional faults further to the south along the major KMF. The Ross Island Thrust is considered to result from deformation ahead of the major northerly propagating Variscan décollement thrust and does not necessitate a continuous décollement structure north of the KMF. Mineralization at Ross Island exhibits remobilization associated with the formation of a pressure-solution cleavage and probably pre-dates thrusting. Copyright © 2003 John Wiley & Sons, Ltd. [source]


    High-Si phengite, mineral chemistry and P,T evolution of ultra-high-pressure eclogites and calc-silicates from the Dabie Shan, eastern China

    GEOLOGICAL JOURNAL, Issue 3-4 2000
    Robert Schmid
    Abstract A suite of coesite,eclogites and associated calc-silicate rocks from the ultra-high-pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high-Si phengite in the eclogites and the enclosing calc-silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are reflected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite-facies rocks, maximum pressure,temperature (P,T) conditions of 40,48 kbar at <,750°C were estimated for the peak-metamorphic mineral assemblages. These P,T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion-controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite- and amphibolite-facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to <,20 kbar at <,600°C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal influence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper-mantle depths. Copyright © 2000 John Wiley & Sons, Ltd. [source]


    Constraints on deformation mechanisms during folding provided by rock physical properties: a case study at Sheep Mountain anticline (Wyoming, USA)

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2010
    K. Amrouch
    SUMMARY The Sheep Mountain anticline (Wyoming, USA) is a well-exposed asymmetric, basement-cored anticline that formed during the Laramide orogeny in the early Tertiary. In order to unravel the history of strain during folding, we carried out combined anisotropy of magnetic susceptibility (AMS), anisotropy of P -wave velocity (APWV) and Fry strain analyses. The results are compared to previously published stress,strain data from calcite twins at the microscopic scale and from fracture sets at the mesoscopic scale, and are used to discuss the kinematics and mechanics of forced folding. The results obtained in sandstone and carbonate lithologies demonstrate a good agreement between (1) the principal axes of the AMS and APWV tensors, (2) stress,strain tensors derived from calcite twins, (3) Fry strain axes and mesoscopic fracture sets. Furthermore, these tensors are coaxial with the main structural trends of the anticline. The differences between AMS and APWV fabrics on one hand, and the differential stress values of the forelimb and the backlimb on the other hand, emphasize how the macroscopic asymmetry of Sheep Mountain anticline affects the strain pattern at the microscopic scale. The data set presented in this paper offers a consistent mechanical scenario for the development of Sheep Mountain anticline. [source]


    Elastic properties of dry clay mineral aggregates, suspensions and sandstones

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2003
    Tiziana Vanorio
    SUMMARY The presence of clay minerals can alter the elastic behaviour of rocks significantly. Although clay minerals are common in sedimentary formations and seismic measurements are our main tools for studying subsurface lithologies, measurements of elastic properties of clay minerals have proven difficult. Theoretical values for the bulk modulus of clay are reported between 20 and 50 GPa. The only published experimental measurement of Young's modulus in a clay mineral using atomic force acoustic microscopy (AFAM) gave a much lower value of 6.2 GPa. This study has concentrated on using independent experimental methods to measure the elastic moduli of clay minerals as functions of pressure and saturation. First, ultrasonic P - and S -wave velocities were measured as functions of hydrostatic pressure in cold-pressed clay aggregates with porosity and grain density ranging from 4 to 43 per cent and 2.13 to 2.83 g cm,3, respectively. In the second experiment, P - and S -wave velocities in clay powders were measured under uniaxial stresses compaction. In the third experiment, P -wave velocity and attenuation in a kaolinite,water suspension with clay concentrations between 0 and 60 per cent were measured at ambient conditions. Our elastic moduli measurements of kaolinite, montmorillonite and smectite are consistent for all experiments and with reported AFAM measurements on a nanometre scale. The bulk modulus values of the solid clay phase (Ks) lie between 6 and 12 GPa and shear (,s) modulus values vary between 4 and 6 GPa. A comparison is made between the accuracy of velocity prediction in shaley sandstones and clay,water and clay,sand mixtures using the values measured in this study and those from theoretical models. Using Ks= 12 GPa and ,s= 6 GPa from this study, the models give a much better prediction both of experimental velocity reduction due to increase in clay content in sandstones and velocity measurements in a kaolinite,water suspension. [source]


    Characterization of cataclastic shear-zones of the KTB deep drill hole by regression analysis of drill cuttings data

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002
    Helmuth Winter
    Summary During the course of the German continental deep drilling project (KTB) two scientific drill holes were drilled, the KTB Vorbohrung down to 4 km and the KTB Hauptbohrung down to 9.1 km, both intersecting several cataclastic shear-zones. As few drill cores were available in the KTB Hauptbohrung, most of the petrophysical and geochemical data are based on drill cuttings investigations. We present an analysis of drill cuttings data, addressing the question of what relationship between cataclastic shear-zones and petrophysical and geochemical data can be revealed. For that purpose we developed a regression model with the amount of cataclastic rocks in drill cuttings as a dependent variable and the petrophysical and geochemical variables as regressors. We use depth related data from two sections of the KTB Hauptbohrung with cataclastic shear-zones in gneiss (1738,2380 m) and in metabasite (4524,4908 m). The variables are selected by estimating and testing a linear regression model taking into account the autocorrelation of the data due to the depth structure. The variables which characterize the cataclastic shear-zones in gneiss according to our model are the contents of carbon and crystal water and the thermal conductivity, each with positive coefficients. This model explains, in total, 57 per cent of the variance of the observed data. For cataclastic shear-zones in metabasite the content of crystal water and the magnetic susceptibility with positive coefficients and the content of chromium with a negative coefficient are the significant variables. The explained variance in this model is 60 per cent. Being significant in both lithologies, the content of crystal water is an important variable for cataclastic shear-zones. The prediction of shear zones is feasible by our methods, but the results of our study should be confirmed and widened by investigations of other data sets. [source]


    Depth distribution of earthquakes in the Baikal rift system and its implications for the rheology of the lithosphere

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001
    Jacques Déverchčre
    Summary The correspondence between the predicted brittle,plastic transition within the crust and the maximum depth of earthquakes is examined in the case of the Baikal rift, Siberia. Although little accurate information on depths is available through large- and moderate-size earthquakes, there are frequent indications of foci at 20 km depth and more. We have relocated 632 events recorded at nearby stations that occurred between 1971 and 1997, with depth and epicentral uncertainties less than 5 km, over the eastern and southern parts of the Baikal rift. We have compared these results with other depth distributions obtained in previous studies from background seismicity in the NE rift (1365 events in the Kalar-Chara zone and 704 events in the Muya region). The relative abundance of earthquakes is generally low at depths between 0 and 10 km (7,15 per cent) and high between 15 and 25 km (,50 per cent). Earthquake activity is still significant between 25 and 30 km (9,15 per cent) and persists between 30 and 40 km (7,13 per cent). Very few earthquakes are below the Moho. We use empirical constitutive laws to obtain the yield-stress limits of several layers made of dominant lithologies and to examine whether the observed distribution of earthquakes at depth (519 events controlled by a close station and located within the extensional domain of the Baikal rift system) can match the predicted crustal strength proportion with depth and the deeper brittle,ductile transition in the crust. A good fit is obtained by using a quartz rheology at 0,10 km depth and a diabase rheology at 10,45 km depth with a moderate temperature field which corresponds to a ,100 Myr thermal lithosphere. No dioritic composition of the crust is found necessary. In any case, earthquakes occur at deep crustal levels, where the crust is supposed to be ductile, in a way very similar to what is found in the East African Rift System. From these results we conclude that the seismogenic thickness is ,35,40 km in the Baikal rift system and that the depth distribution of earthquakes is at first order proportional to the strength profile found in a rheologically layered crust dominated by a mafic composition in the ,10,45 km depth range. An upper mantle core with high strength, however, generally prevents it from reaching stress failure at greater depth. [source]


    Magnetic polarity stratigraphy of Siwalik Group sediments of Karnali River section in western Nepal

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2000
    Pitambar Gautam
    The remanent magnetization of siltstones and sandstones sampled at 476 levels/sites throughout a 3560 m thick molasse sequence belonging to the Siwalik Group (0,2015 m: Lower; 2015,3560 m: Middle) has been studied by stepwise thermal demagnetization. This section is exposed along the Karnali River in Nepal. The natural remanent magnetization (NRM) usually consists of two components: a viscous or thermoviscous component of recent field origin, and an ancient characteristic component (ChRM). The former component is of normal polarity and resides either in goethite (unblocking temperature < 150 °C; resistant to AFD up to 150 mT) or in maghemite (unblocking temperature 150,400 °C). Goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section. Maghemite content is significant in the grey mud-free lithologies from the upper half of the section. The main component, unblocked in the high-temperature range (commonly 610,680 °C) and believed to reside in haematite, presumably of mostly detrital origin, represents a characteristic remanence (ChRM). The tilt-corrected ChRM directions at individual sites show antipodal clusters (ratio of normal- to reverse-polarity sites: 0.62), and yield mean inclinations recording significant inclination shallowing,a feature well recorded in the Siwaliks. This ChRM is interpreted to represent a largely primary detrital remanence. The ChRM data from 430 sites yield the Karnali River magnetic polarity sequence, whose correlation with the geomagnetic polarity timescale (Cande & Kent 1995) suggests a depositional age of 16 Ma (younger than chron C5Cn.1n) to 5.2 Ma (around the top of chron C3r) for the 3560 m section sampled. Hence, the Karnali River exposes the oldest part of the Siwalik Group in Nepal. Estimates of the sediment accumulation rate (SAR) average to 32.9 cm kyr,1 for the 10.8 Myr time span of deposition. [source]


    Interpretation of Spring Recession Curves

    GROUND WATER, Issue 5 2002
    H. Amit
    Recession curves contain information on storage properties and different types of media such as porous, fractured, cracked lithologies and karst. Recession curve analysis provides a function that quantitatively describes the temporal discharge decay and expresses the drained volume between specific time limits (Hall 1968). This analysis also allows estimating the hydrological significance of the discharge function parameters and the hydrological properties of the aquifer. In this study, we analyze data from perennial springs in the Judean Mountains and from others in the Galilee Mountains, northern Israel. All the springs drain perched carbonate aquifers. Eight of the studied springs discharge from a karst dolomite sequence, whereas one flows out from a fractured, slumped block of chalk. We show that all the recession curves can be well fitted by a function that consists of two exponential terms with exponential coefficients ,1 and ,2. These coefficients are approximately constant for each spring, reflecting the hydraulic conductivity of different media through which the ground water flows to the spring. The highest coefficient represents the fast flow, probably through cracks, or quickflow, whereas the lower one reflects the slow flow through the porous medium, or baseflow. The comparison of recession curves from different springs and different years leads to the conclusion that the main factors that affect the recession curve exponential coefficients are the aquifer lithology and the geometry of the water conduits therein. In normal years of rainy winter and dry summer, ,1 is constant in time. However, when the dry period is longer than usual because of a dry winter, ,1 slightly decreases with time. [source]


    Environmental isotopic and hydrochemical characteristics of groundwater systems in Daying and Qicun geothermal fields, Xinzhou Basin, Shanxi, China

    HYDROLOGICAL PROCESSES, Issue 22 2010
    Dongmei Han
    Abstract The conceptual hydrogeological model of the low to medium temperature Daying and Qicun geothermal fields has been proposed, based on hydrochemical characteristics and isotopic compositions. The two geothermal fields are located in the Xinzhou basin of Shanxi, China and exhibit similarities in their broad-scale flow patterns. Geothermal water is derived from the regional groundwater flow system of the basin and is characterized by Cl·SO4 -Na type. Thermal water is hydrochemically distinct from cold groundwater having higher total dissolved solids (TDS) (>0·8 g/l) and Sr contents, but relatively low Ca, Mg and HCO3 contents. Most shallow groundwater belongs to local flow systems which are subject to evaporation and mixing with irrigation returns. The groundwater residence times estimated by tritium and 14C activities indicate that deep non-thermal groundwater (130,160 m) in the Daying region range from modern (post-1950s) in the piedmont area to more than 9·4 ka BP (Before Present) in the downriver area and imply that this water belong to an intermediate flow system. Thermal water in the two geothermal fields contains no detectable active 14C, indicating long residence times (>50 ka), consistent with this water being part of a large regional flow system. The mean recharge elevation estimated by using the obtained relationship Altitude (m) = , 23·8 × ,2H (, ) , 121·3, is 1980 and 1880 m for the Daying and Qicun geothermal fields, respectively. The annual infiltration rates in the Daying and Qicun geothermal fields can be estimated to be 9029 × 103 and 4107 × 103 m3/a, respectively. The variable 86Sr/87Sr values in the thermal and non-thermal groundwater in the two fields reflect different lithologies encountered along the flow path(s) and possibly different extents of water-rock interaction. Based on the analysis of groundwater flow systems in the two geothermal fields, hydrogeochemical inverse modelling was performed to indicate the possible water-rock interaction processes that occur under different scenarios. Copyright © 2010 John Wiley & Sons, Ltd. [source]


    Temporal and spatial variability of cation and silica export in an alpine watershed, Emerald Lake, California

    HYDROLOGICAL PROCESSES, Issue 10 2004
    T. Meixner
    Abstract A reaction set of possible mineral weathering reactions is proposed to explain observed cation and silica export for the Emerald Lake watershed, a small Sierra Nevada, California catchment. The reaction set was calculated through a stoichiometric mole-balance method, using a multiyear record of stream flow and snowpack chemical analyses and site-specific mineral compositions. Reaction-set calculations were intended to explore how the processes controlling stream cation and silica export depend on differing bedrock mineralogy across the catchment as snowmelt and runoff patterns change over the year. Different regions within the watershed can be differentiated by lake inflow subdrainages, each exhibiting different stream-flow chemistry and calculated weathering stoichiometry, indicating that different silica and cation generation processes are dominant in wet steep portions of the catchment. Short-term differences in stream concentrations were assumed to reflect ion exchange equilibria and rapid biological processes, whereas long-term persistent stream concentration differences in different areas of the catchment were assumed to reflect spatial variability in mineral weathering stoichiometry. Mineralogical analyses of rock samples from the watershed provided site-specific chemical compositions of major mineral species for reaction calculations. Reaction sets were evaluated by linear regression of calculated versus observed differences between snowmelt and stream-flow chemistry and by a combined measure. Initially, single weathering reactions were balanced and evaluated to determine the reactions that best explained observed stream chemical export. Next, reactions were combined, using mineral compositions from different rock types to estimate the dependence of ion fluxes on lithology. The seasonal variability of major solute calculated fluxes is low, approximately one order of magnitude, relative to the observed three orders of magnitude variability in basin discharge. Reaction sets using basin-averaged lithology and Aplite lithologies gave superior explanations of stream chemical composition. Copyright © 2004 John Wiley & Sons, Ltd. [source]


    Baguio Mineral District: An oceanic arc witness to the geological evolution of northern Luzon, Philippines

    ISLAND ARC, Issue 4 2008
    Graciano P. Yumul Jr
    Abstract The Baguio Mineral District exposes rock formations that evince the geological and tectonic evolution of this district from a subduction-related marginal basin to an island arc setting. Available onshore and offshore data are consistent with an Early (onset phase) to Middle (developed phase) Miocene arc polarity reversal from the east (termination of subduction along the proto-East Luzon Trough) to the west (initiation of subduction along the Manila Trench). Geophysical modeling and geochemical data calculation showed a 30 ± 5 km crustal thickness for the mineral district. Subduction-related multiple arc magmatism and ophiolite accretion contributed to crustal thickening. Recent information on the Oligo,Miocene Zigzag and Klondyke formations in the mineral district reveal that the marginal basin, where these rocks were deposited, has received eroded materials from adjacent terrains characterized by siliceous lithologies. Furthermore, adakitic rocks, high permeable zones and extensional zones which are exploration markers applied to identify possible mineralization targets, are prevalent in the mineral district. The geological evolution that the district had undergone mimics the evolution of island arcs worldwide in general and northern Luzon in particular. [source]


    Tectono-metamorphic history of the Tacagua ophiolitic unit (Cordillera de la Costa, northern Venezuela): Insights in the evolution of the southern margin of the Caribbean Plate

    ISLAND ARC, Issue 1 2007
    Alessandro Ellero
    Abstract The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid-oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep-sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4. Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel (D2 and D4 phases) and normal (D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike-slip and pure compressional or pure extensional tectonics occurred through time. [source]


    Overview of the geology, petrology and tectonic framework of the high-pressure,ultrahigh-pressure metamorphic belt of the Kokchetav Massif, Kazakhstan

    ISLAND ARC, Issue 3 2000
    S. Maruyama
    Abstract High- to ultrahigh-pressure metamorphic (HP,UHPM) rocks crop out over 150 km along an east,west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet-like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north-dipping, fault-bounded orogen-parallel units (I,IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high-pressure amphibolite through quartz,eclogite and coesite,eclogite to diamond,eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780,1000°C, P = 37,60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730,750°C, P = 11,14 kbar; Unit IV: T = 530°C, P = 7.5,9 kbar) and bottom (Unit I: T = 570,680°C; P = 7,13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top-to-the-north) and top (top-to-the-south) of the pile. The orogen-scale architecture of the massif is sandwich-like, with the HP,UHPM nappe group juxtaposed across large-scale subhorizontal faults, against underlying low P,T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP,UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower-grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30,40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab-breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP,UHPM rocks. [source]


    Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, Japan

    ISLAND ARC, Issue 1 2000
    Daniel K. Asiedu
    Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high-grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote-amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle-derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted. [source]


    Experimental studies of mineralogical assemblages of metasedimentary rocks at Earth's mantle transition zone conditions

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2007
    L. F. DOBRZHINETSKAYA
    Abstract Metasedimentary rocks, a major component of the continental crust, are abundant within ultra-high pressure (UHP) metamorphic terranes related to continental collisions. The presence of diamond, coesite, and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150,250 km. Reconnaissance experiments at 9,12 GPa and 1000,1300 °C on compositions corresponding to felsic rocks from diamond-bearing UHP terranes of Germany and Kazakhstan show that at higher pressures they consist of majoritic garnet, Al-Na-rich clinopyroxene, stishovite, solid solution of KAlSi3O8 -NaAlSi3O8 hollandite, topaz-OH, and TiO2 with , -PbO2 structure. Comparison of our data with experiments conducted by others at similar P,T conditions shows differences, which are due to variations in bulk chemistry and the type of starting material (gel, oxides, minerals). These differences may affect correct establishment of the ,point of no return' of subducted continental lithologies. This paper discusses the implication of the experimental data with regard to naturally existing UHP metamorphic rocks and their significance for our understanding of the deep subduction of continental material. [source]


    Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic,ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2004
    A. Essaifi
    Abstract During emplacement and cooling, the layered mafic,ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca-amphibole, Mg-chlorite and CaAl-silicates. In the zones of active deformation (centimetre-scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca-amphibole-rich matrix. Mass-balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature. When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre-scale shear zones with time. The inner parts of the shear zones were transformed into chlorite-rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca,Al silicates, while calcite and quartz were deposited in ,en echelon' veins. Mass-balance calculations indicate that formation of the chlorite-rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies. [source]


    Hydrothermal alteration of late- to post-tectonic Lyon Mountain Granitic Gneiss, Adirondack Mountains, New York: Origin of quartz,sillimanite segregations, quartz,albite lithologies, and associated Kiruna-type low-Ti Fe-oxide deposits

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2002
    J. Mclelland
    Abstract Quartz,sillimanite segregations, quartz,albite lithologies (Ab95,98), and Kiruna-type low-Ti iron-oxide deposits are associated with late- to post-tectonic (c. 1055 Ma) leucogranites of Lyon Mountain Gneiss (LMG) in the Adirondack Mountains, New York State. Most recent interpretations of these controversial features, which are global in occurrence, favour hydrothermal origins in agreement with results presented here. Field relations document that quartz,sillimanite veins and nodules cut, and therefore post-date, emplacement of host LMG leucogranites. Veins occur in oriented fracture networks, and aligned trains of nodules are interpreted as disrupted early veins. Late dykes of leucogranite cut veins and nodules demonstrating formation prior to terminal magmatism. Veins and nodules consist of sillimanite surrounded by quartz that commonly embays wall-rock feldspar indicating leaching of Na and K from LMG feldspar by acidic hydrothermal fluids. Subsequent, and repeated, ductile flow disrupted earlier veins into nodular fragments but produced little grain shape fabric. Geochemical and petrographic studies of quartz,albite rock indicate that it formed through metasomatic replacement (albitization) of LMG microperthite by sodic hydrothermal fluids that resulted in diagnostic checkerboard albite. Low-Ti iron-oxide ores are commonly associated with the quartz,albite sub-unit, and it is proposed that hydrothermal fluids related to albitization transported Fe as well. The regional extent of sodic alteration suggests large quantities of surface-derived hydrothermal fluids. Fluid inclusion and oxygen isotope data are consistent with high temperature, regionally extensive fluids consisting primarily of evolved surface-derived brines enriched in Na and Cl. Quartz,sillimanite veins and nodules, which are significantly more localised phenomena and require acidic fluids, were most likely formed from local magmatic fluids in the crystallizing carapaces of LMG plutons. [source]


    PETROPHYSICAL CHARACTERISTICS OF SOURCE AND RESERVOIR ROCKS IN THE HISTRIA BASIN, WESTERN BLACK SEA

    JOURNAL OF PETROLEUM GEOLOGY, Issue 4 2009
    C. Cranganu
    The petroleum system in the Histria Basin, Western Black Sea, includes Oligocene source rocks and Upper Cretaceous , Eocene reservoir rocks. Here we report on the petrophysical characteristics of these source and reservoir rocks using mercury intrusion porosimetry data from 14 core samples collected from five wells drilled on the East Lebada, West Lebada and Pescarus structures. Samples were in general dominated by carbonate lithologies with minor shales. Petrophysical parameters analyzed were: median pore-throat radius, average pore-throat radius, apparent porosity, pore-throat size distribution, pore-throat type, pore-throat sorting, maximum threshold entry radius, pore-throat radius at 35% mercury saturation (R35), and air permeability. Reservoir rock quality was estimated using a permeability / porosity / pore-throat type plot. The Oligocene samples showed little petrophysical variation. Samples were relatively homogenous and had the same pore-throat type (nano), were well sorted, had unimodal pore-throat distribution (suggesting the existence of a single fluid phase), had similar values for median and average pore-throat radius, and similar values for R35 and maximum threshold entry radius. Upper Cretaceous , Eocene samples were more heterogeneous in terms of petrophysical properties, and reservoir quality was in general higher than in the Oligocene interval. Average porosity and calculated air-permeability values were 18.4% and 0.37 mD, respectively for Upper Cretaceous samples; and 11.8% and 27.11 mD, respectively for Eocene samples. A case study of Oligocene and Cretaceous , Eocene samples from well West Lebada 817 is presented. This paper represents the first petrophysical study of source and reservoir rocks in the Histria Basin, Western Black Sea. The results will help to establish the links between petrophysical characteristics, age and depositional environment for source and reservoir rocks in other basins bordering the Black Sea. [source]


    IMPACT OF COAL SURFACE MINING AND RECLAMATION ON SUSPENDED SEDIMENT IN THREE OHIO WATERSHEDS,

    JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 4 2000
    James V. Bonta
    ABSTRACT: Prior to PL95,87 little research had been conducted to determine the impacts of mining and reclamation practices on sediment concentrations and yields on a watershed scale. Furthermore, it was unknown whether sediment yield and other variables would return to undisturbed levels after reclamation. Therefore, three small watersheds, with differing lithologies and soils, were monitored for runoff and suspended sediment concentrations during three phases of watershed disturbances: undisturbed watershed condition, mining and reclamation disturbances, and post-reclaimed condition. Profound increases in suspended-sediment concentrations, load rates, and yields due to mining and reclamation activities, and subsequent drastic decreases after reclamation were documented. Even with increases in runoff potential, reductions in suspended-sediment concentrations and load rates to below or near undisturbed-watershed levels is possible by using the mulch-crimping technique and by removing diversions. Maximum concentrations and load rates occurred during times of active disturbances that exposed loose soil and spoil to high-intensity rains. Sediment concentrations remained elevated compared with the undisturbed watershed when diversions were not well maintained and overtopped, and when they were not removed for final reclamation. Diversions are useful for vegetation establishment, but should be maintained until they are removed for final reclamation after good vegetative cover is established. [source]


    Quaternary landscape evolution: a framework for understanding contemporary erosion, southeast Spain

    LAND DEGRADATION AND DEVELOPMENT, Issue 2 2002
    A. E. Mather
    Abstract Recent research into the long-term landscape development of a tectonically active terrain in arid SE Spain has revealed the significance of river capture in understanding current landscape instabilities (badlands and landslides). The river capture was initiated at c.100,ka,BP and effected a 90,m base-level change at the point of capture. This stimulated a wave of incision to propagate through the landscape to 20,km upstream of the capture site. The net effect of the associated increase in erosion has been to change valley shapes from broad and shallow to narrow and deep. The associated unloading and steepening of valley sides has led to a focus of landslide activity in lithologies with more unconfined compressive strength (limestones) and a dominance of gullying, piping and badland development in the lithologies with lower unconfined compressive strengths (marls and sands). Post-capture rapid valley widening was initially achieved through landslide development. This form of slope degradation was sustained in the more resistant, joint-controlled lithologies. In weaker lithologies it was superseded by badland development. The elevated sediment fluxes associated with the c.100,ka,BP base-level perturbation will continue into the near future, but are expected to decay, assuming that no additional environmental disturbances occur. The patterns of landscape instability witnessed today are controlled by (1) proximity to the areas affected by the base-level change and (2) the robustness of the local geology. Understanding of this long-term temporal context of the landscape provides a valuable spatial and temporal framework for land system management, facilitating the prediction of future natural trends in landscape stability. Copyright © 2002 John Wiley & Sons, Ltd. [source]


    Effects of diagenesis on the astrochronological approach of defining stratigraphic boundaries in calcareous rhythmites: The Tortonian GSSP

    LETHAIA, Issue 4 2008
    HILDEGARD WESTPHAL
    Since the establishment of astrochronology, calcareous rhythmites are frequently used as the basis of high-resolution chronostratigraphy. In particular for the Neogene, calcareous rhythmites serve as stratotypes and for absolute dating of stratigraphic boundaries (Global Stratotype Sections and Points , GSSPs). However, the exact mechanisms responsible for the formation of the rhythmic intercalation of lithologies in such successions are complex and not easily reconstructed. To a large extent this is the effect of diagenetic modifications of the original sediment. Here, two examples of Neogene calcareous rhythmites are studied; the Monte dei Corvi section and the Monte Gibliscemi section. The first is the GSSP location of the Serravallian,Tortonian boundary, whereas the second is an auxiliary stratotype for the same boundary. During the past years, astrochronological approaches were applied to these successions to considerably increase time resolution compared to the elaborated biostratigraphic database. The present study focuses on micropetrographic, trace element and clay mineralogical methods in order to gain a better understanding of the genesis of the rhythmites. In the Monte Gibliscemi section, sediment parameters that are robust against diagenetic change clearly reflect primary differences, i.e. cyclical environmental changes. In contrast, no clear primary signal is determined for the Monte dei Corvi section on a couplet scale, leaving the origin of the rhythm ambiguous. This impedes the interpretation of the latter and the comparability between the two successions, and also compromises any bed-by-bed correlation between the two. The unclear origin of the rhythmites of Monte dei Corvi introduces uncertainty into the applicability of astrochronology to this succession. [source]


    Compositional and lithological diversity among brecciated lunar meteorites of intermediate iron concentration

    METEORITICS & PLANETARY SCIENCE, Issue 9 2009
    Randy L. Korotev
    Most have iron concentrations intermediate to those of the numerous feldspathic lunar meteorites (3,7% FeO) and the basaltic lunar meteorites (17,23% FeO). All but one are polymict breccias. Some, as implied by their intermediate composition, are mainly mixtures of brecciated anorthosite and mare basalt, with low concentrations of incompatible elements such as Sm (1,3 ,g/g). These breccias likely originate from points on the Moon where mare basalt has mixed with material of the FHT (Feldspathic Highlands Terrane). Others, however, are not anorthosite-basalt mixtures. Three (17,75 ,/g Sm) consist mainly of nonmare mafic material from the nearside PKT (Procellarum KREEP Terrane) and a few are ternary mixtures of material from the FHT, PKT, and maria. Some contain mafic, nonmare lithologies like anorthositic norites, norites, gabbronorites, and troctolite. These breccias are largely unlike breccias of the Apollo collection in that they are poor in Sm as well as highly feldspathic anorthosite such as that common at the Apollo 16 site. Several have high Th/Sm compared to Apollo breccias. Dhofar 961, which is olivine gabbronoritic and moderately rich in Sm, has lower Eu/Sm than Apollo samples of similar Sm concentration. This difference indicates that the carrier of rare earth elements is not KREEP, as known from the Apollo missions. On the basis of our present knowledge from remote sensing, among lunar meteorites Dhofar 961 is the one most likely to have originated from South Pole-Aitken basin on the lunar far side. [source]


    The effect of target lithology on the products of impact melting

    METEORITICS & PLANETARY SCIENCE, Issue 12 2008
    G. R. OSINSKI
    Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or "sheets" are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impact-generated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater-fill impactites) for impacts into sedimentary-bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary. [source]